1. Field of the Invention
The present invention relates to a process for producing in a simple manner a clay mineral of chain structure such as sepiolite and attapulgite.
2. Description of the Related Art
A clay mineral of chain structure such as sepiolite and attapulgite finds use as, for example, a catalyst for chemical reactions, an adsorbent for chromatography, a membrane for separating various mixtures, and a rheology controlling material.
Sepiolite is a fibrous clay mineral represented by the general formula below. EQU (Mg.sub.8-y-z R.sub.y.sup.3+ X.sub.z)Si.sub.12-x R.sub.x.sup.3+)O.sub.30 (OH).sub.4 (OH.sub.2) .sub.4 Q.sup.2+ (x-y+2z)/2.multidot.(H.sub.2 O).sub.8
where R denotes at least one member selected from Al and Fe; Q denotes Ca; X denotes a lattice defect; and x, y, and z denote the substitution of tetrahedral ion, the substitution of octahadral ion, and the defect of octahedral ion, respectively.
It has channels (measuring 3.7 .ANG..times.9.3 .ANG.) running parallel to the direction of the fiber. In these channels are bound water molecules coordinating to Mg.sup.2+. Owing to this structure, sepiolite is used as an adsorbent and drilling mud for geothermal exploitation.
The clay mineral of chain structure now in use is of natural origin because of difficulty in its synthesis. However, there are some reports on the synthesis. According to B. Siffert and R. Wey (Comptes rendes vol. 254, pp. 1460-1464, 1962), sepiolite is synthesized when sodium hydroxide is added to silicic acid and magnesium chloride, followed by standing at room temperature for 3 weeks. The reaction product formed in this manner is mostly amorphous and sepiolite would be very small in quantity even though it should occur. N. I. Nesterchuk and T. A. Makarova reported a process of synthesizing sepiolite by the reaction of magnesium chloride with sodium metasilicate, followed by the heat treatment in an autoclave at 250.degree. C. for 3-5 hours. (Zap. Vses. Mineral Obshchest, 1973, 102(2), 232-4) This process, however, merely yields a product of poor crystallinity as indicated by powder X-ray diffraction. Thus it is difficult to produce sepiolite efficiently.